The present technology relates to a holographic data storage medium which can be used, for example, for storing image data such as photos, logos, text, and so on but also for the storage of other data.
In a hologram, optical phase information about an object is contained distributed over the surface of the hologram, from which an image of the object can be reconstructed when it is irradiated with light, in particular coherent light from a laser. Holograms are used in industry in many ways, for example in the form of largely counterfeit-proof identifications. Identifications of this type will be found, for example, on credit cards or cheque cards; as what are known as white light holograms, they show a three-dimensional image of the object represented even when lit with natural light. Photographically produced holograms and embossed holograms are widespread, in which a relief structure is embossed into the surface of a material, at which the light used to reproduce the object is scattered in accordance with the information stored in the hologram, so that the reconstructed image of the object is produced by interference effects.
WO 00/17864 describes a data storage medium having an optical information carrier which contains a polymer film set up as a storage layer. The polymer film consists, for example, of biaxially oriented polypropylene. In the previously disclosed data storage medium, the polymer film is wound spirally in a plurality of layers onto a core, there being an adhesive layer in each case between adjacent layers. Information can be written into the data storage medium by the polymer film being heated locally with the aid of a write beam, focused on a predetermined position, from a data drive, as a result of which the refractive index of the polymer film and the reflection capacity at the interface of the polymer film change locally. This can be registered with the aid of a read beam in the data drive, since the read beam is reflected locally more or less intensesly at the interface of the polymer film, depending on the information written in.
The present technology provides a holographic data storage medium which is cost-effective and has wide possible applications.
The holographic data storage medium according to an exemplary embodiment of the invention has a polymer film which is set up as a storage layer and whose surface structure can be changed locally by heating. The polymer film is set up for the storage of holographic information via the local surface structure of the polymer film.
The surface structure or topography of the polymer film may be changed locally by, for example, a laser beam serving as a write beam being focused onto the polymer film, preferably its surface zone, so that the light energy is absorbed there and converted into heat energy. In particular if the laser beam is radiated in briefly (pulsed), the material change in the polymer film leading to the local change in the surface structure is limited to a very small volume owing to the generally poor thermal conductivity of the polymer.
When information is put into the holographic data storage medium, the structure change of the surface is preferably induced point by point, as explained in more detail further below. The local region for the storage of a unit of information (referred to in the following text as a “pit”) typically has linear lateral dimensions (that is to say for example a side length or a diameter) of the order of magnitude of 0.5 μm to 1 μm, although other sizes are possible. The height profile of the polymer film, in the event of a change in the surface structure in a pit, typically changes by 50 nm to 500 nm, which depends in detail on the characteristics and operating conditions of the write beam and the characteristics of the data storage medium. The point grid, that is to say the centre spacing between two pits, typically lies in the range from 1 μm to 2 μm. It is generally true that shorter optical wavelengths of the write beam permit a closer point grid.
In order to read information out of the holographic data storage medium according to an exemplary embodiment of the invention, the storage layer can be irradiated with coherent light, so that the surface structure of the polymer film acts like a diffraction grating and a holographic image is produced by interference effects, as explained in more detail further below.
The holographic data storage medium according to the present technology is cost-effective and may be applied in many ways.
The polymer film can be oriented and is preferably biaxially oriented, for example by being prestressed in two directions at right angles to each other within its plane during production. In the case of an oriented polymer film, a high energy density is stored in the film material. As a result of local heating with the deposition of a relatively small quantity of energy per unit area, for example with the aid of a write beam, a relatively intense material change with a change in the local surface structure of the polymer film can be achieved. Biaxially oriented polymer films may be produced from cost-effective mass-produced plastics.
Suitable materials for the polymer film are, for example, polypropylene or polyvinyl chloride, polymer films which have such a material preferably being biaxially oriented. A higher temperature stability and therefore also an improved resistance to ageing and storage stability of the holographic data storage medium, and increased security against loss of data arising from ageing processes may be achieved with polymer films which have an elevated crystallite melting point. Examples of such materials are polyethylene terephthalate (PET), polymethyl pentene (PMP); also poly-2-methyl pentene) and polyimide, a polymer film of such materials also preferably being biaxially oriented. In the case of higher intensities of a write beam, other types of film can also be used.
Preferred thicknesses of the polymer film lie in the range from 10 μm to 200 μm, but smaller or larger thicknesses are also conceivable.
To the polymer film, there can be assigned an absorber dye, which is set up to at least partly absorb a write beam serving to input information and to give up the heat produced in the process at least partly locally to the polymer film. An absorber dye of this type permits local heating of the polymer film which is sufficient to change the surface structure at a relatively low intensity of the write beam. The absorber dye is preferably arranged in the material of the polymer film. However, it can also be arranged in a separate absorber layer, which preferably has a binder; mixed forms are likewise conceivable. For example, the absorber layer can have a thin layer (for example a thickness of 1 μm to 5 μm) of an optically transparent polymer (for example of polymethyl methacrylate PMMA) or, in the case of applications for higher temperature, of polymethyl pentene, polyether etherketone (PEEK) or polyetherimide) which serves as a matrix or binder for the molecules of the absorber dye. The absorption maximum of the absorber dye should coincide with the optical wavelength of the write beam used, in order to achieve efficient absorption. For an optical wavelength of 532 nm of a write beam produced by a laser, for example dyes from the Sudan red family (diazo dyes) or (for particularly polar plastics) eosin scarlet are suitable. For the common laser diodes with an optical wavelength of 665 nm or 680 nm, green dyes, for example from the styryl family (which are commonly used as laser dyes), are more suitable.
A reflective layer can be arranged on that surface of the polymer film which is changed during the storage of holographic information. If this reflective layer is applied after information has been put in, or is not changed or not significantly changed when information is put in, it increases the efficiency of the holographic data storage medium. This is because coherent light used when information is being read out is to a large extent reflected at the surface by the reflective layer and, in the process, modulated by the surface structure, so that a reflectively bright holographic image is produced.
However, the reflective layer can also be configured in such a way that the reflection capacity (reflectivity) of the reflective layer can be changed locally by heating. This may be achieved, for example, with a very thin reflective layer which, under the action of heat from a write beam, is locally partly or completely evaporated. A reflective layer of this type permits a holographic data storage medium in which both the surface structure of the polymer film (and also of the adjacent reflective layer) and the reflection capacity or the optical transmissivity of the reflective layer are changed when information is put in. When information is read out, in addition to phase modulation (because of the varying surface structure), amplitude modulation (because of the varying optical transmissivity) is consequently also obtained. A holographic data storage medium of this type offers numerous possible applications.
In an alternative refinement of the holographic data storage medium, a reflective layer is arranged on that surface of the polymer film which is opposite the surface that is varied during the storage of holographic information. This reflective layer is preferably already present before information is put in and is not changed when information is put in. It is preferably flat and effects reflection of light, which is used to read out information after it has passed through the varying surface structure of the polymer film and the polymer film itself. One advantage of such a reflective layer is the possibility of being able to apply the holographic data storage medium to a nontransparent carrier or virtually any desired substrate. The thickness of the polymer film serving as a storage layer is preferably selected such that no disturbed interference or superposition effects occur.
In a preferred refinement of the present technology, the holographic data storage medium has an adhesive layer for sticking the data storage medium to an object. The adhesive layer makes it possible to stick the data storage medium quickly and without difficulty to a desired object, for example to use the data storage medium as a machine-readable label in which information about the object is stored. Particularly suitable as an adhesive layer is a self-adhesive layer or a layer with a pressure-sensitive adhesive, which, in the delivered state of the data storage medium, is preferably provided with a protective covering that can be pulled off (for example of a film or a silicone paper).
Apart from the layers previously mentioned, the data storage medium according to the present technology can also have additional layers, for example a transparent protective layer or a flexible or dimensionally stable carrier.
The information to be stored can be input into the holographic data storage medium according to the present technology by a method in which holographic information contained in a hologram of a storing object is calculated as a two-dimensional arrangement, and a write beam of a writing device, preferably a laser lithograph, is aimed at a storage layer and/or if appropriate the associated absorber layer of the data storage medium and is driven in accordance with the two-dimensional arrangement in such a way that the local surface structure of the polymer film set up as a storage layer is set in accordance with the holographic information. Since the physical processes in the scattering of light at a storing object are known, for example a conventional set-up for producing a hologram (in which coherent light from a laser, which is scattered by an object (storing object) is brought into interference with a coherent reference beam and the interference pattern produced in the process is recorded as a hologram) is simulated with the aid of a computer program, and the interference pattern or the modulation for the surface structure of the polymer film is calculated as a two-dimensional arrangement (two-dimensional array). The resolution of a suitable laser lithograph is typically about 50 000 dpi (dots per inch). The surface structure of the polymer film can therefore be changed locally in regions or pits with a size of about 0.5 μm to 1 μm. The writing speed and other details depend, inter alia, on the parameters of the write laser (laser power, optical wavelength) and the time of exposure and on the characteristics of the storage layer and any absorber dye.
The holographic information is therefore preferably put in in the form of pits of predefined size; the term “pit” is to be understood here in general terms in the sense of a changed region and not exclusively in its original meaning as a hole or depression. In this case, the holographic information can be stored in a pit in binary encoded form. This means that, in the region of a given pit, the surface structure of the storage layer assumes only one of two possible basic shapes. These basic shapes preferably differ considerably in order that intermediate forms which occur in practice and which are close to one or the other basic shape can be assigned unambiguously to one or the other basic shape, in order to store the information reliably and unambiguously.
Alternatively, the holographic information can be stored in a pit in continuously encoded form, the local maximum height change in the surface structure in the pit being selected from a predefined value range. This means that, in a given pit, the surface structure of the storage layer can assume intermediate shapes between two basic shapes, so that the maximum height change in the present intermediate shape assumes a value from a predefined value range whose limits are given by the maximum height changes of the two basic shapes. In this case, the information can therefore be stored “in grey stages”, so that each pit is given the information content from more than one bit.
If a reflective layer is arranged on the surface of the polymer film that is changed during the storage of holographic information, this reflective layer is preferably applied after the local surface structure of the polymer film has been set in accordance with the holographic information. However, the reflective layer can also be applied before the local surface structure of the polymer film is set in accordance with the holographic information. In the latter case, a reflective layer whose reflection capacity can be changed locally by heating permits a holographic data storage medium in which both the local surface structure of the polymer film and the local reflection capacity of the reflective layer can be set in accordance with the holographic information via local heating by means of the write beam. The advantages of these variants are explained further above.
In a method of reading information out of a holographic data storage medium according to the present technology, light, preferably coherent light (for example from a laser), is aimed at a large area of a storage layer of the data storage medium and modulated by the surface structure of the polymer film. As a reconstruction of the holographic information contained in the region covered by the light, a holographic image is registered at a distance from the data storage medium, for example by a CCD sensor which is connected to a data processing device.
The term “large area” is to be understood to mean an area which is considerably larger than the area of a pit. In this sense, for example, an area of 1 mm.sup.2 is a large area. For the scheme according to which information is stored in a holographic data storage medium according to the present technology and read out, there are many different possibilities. It is conceivable to read out from the data storage medium in one operation, by the entire area of the polymer film set up as a storage layer being illuminated in one operation. In particular in the case of larger areas, however, it is advantageous to divide up the information to be stored into a number or large number of individual regions (for example with a respective area of 1 mm.sup.2) and to read out the information only from a predefined individual area in one operation.
The fact that during the reading of information, depending on the configuration of the holographic data storage medium, the light shines through the polymer film or else does not shine through the polymer film (if it is reflected directly at the surface structure or at a reflective layer located there) and may possibly be modulated not only by the surface structure of the polymer film but also by the locally varying reflection capacity of the reflective layer, has already been explained further above.
During the reading of information, as a result of the locally varying surface structure of the polymer film, that is to say its regionally different topography, differences occur in the propagation times of the light waves which originate from various points, that is to say ultimately periodic phase modulation. This applies both to arrangements in which the polymer film is transilluminated (with or without reflection) and for arrangements in which it is not transilluminated (direct reflection at the surface structure). Added to this, in the case of a locally varying reflection capacity, is an amplitude modulation. The region of the polymer film covered by the coherent light acts like a diffraction grating which deflects incident light in a defined manner. The deflected light forms an image of the stored object, which represents the reconstruction of information encoded in the holographic data storage medium.
The holographic data storage medium according to the present technology can be used for different types of storing objects. For example, both the information contained in images, such as photographs, logos, text, and so on, and machine-readable data can be stored and read out. The latter is carried out, for example, in the form of data pages, as they are known, the holographic information contained in a hologram of a graphic bit pattern (which represents the data information) being input into the storage layer as explained. When the said data is read out, a holographic image of this graphic bit pattern is produced. The information contained therein can be registered, for example with the aid of an accurately adjusted CCD sensor, and processed by associated evaluation software. For the reproduction of images, in which high accuracy is not an issue, in principle even a simple matt disc, or, for example, a camera with an LCD screen is sufficient.
In the case of the holographic storage of machine-readable data, it is advantageous that the information does not have to be read out sequentially but that an entire data set can be registered in one operation, as explained. Should the surface of the storage layer be damaged, then, as opposed to a conventional data storage medium, this does not lead to a loss of data but only to a worsening of the resolution of the holographic image reconstructed when the information is read out, which is generally not a problem.